The invention relates to a carbonate precursor material of a lithium nickel manganese cobalt oxide with high lithium and manganese content, to be used as a cathode material for Li-ion batteries. The present invention aims at providing cathode materials which exhibit good electrochemical performances, by using certain carbonate precursors that are sintered with a lithium precursor for forming the cathode material.
In present and future applications, Li batteries with high energy density are needed. A high energy density can be achieved by cathodes having either one or (preferably) both of a high volumetric density and a high specific reversible discharge capacity. For a long time LiCoO2 (or “LCO”) was the dominating cathode material for rechargeable lithium batteries. LCO has a relatively high capacity (150-160 mAh/g when cycled at 3-4.3V) together with a high density (the true density is about 5.05 g/cm3) and is relatively easy to produce. It has a relatively high Li diffusion, so it is possible to utilize large and dense particles (10-20 μm size) with a small surface area (0.1-0.5 m2/g). These large, dense, low surface area particles can easily be prepared with a small amount of soluble surface base. These bases are originating mainly from two sources: first, impurities such as Li2CO3 and LiOH present in the Li-M-O2; second, bases originating from ion exchange at the powder surface: LiMO2+δH+←→Li1−δHδMO2+δLi+. All in all, commercial LCO is a robust and easy to manufacture cathode powder.
LCO however also has serious drawbacks. A main drawback is the relative scarcity of Co resources related to the relatively high cost of cobalt metal. Still worse, historically the cobalt price shows wild fluctuations, and these fluctuations possibly increased the need to find substitutes for LiCoO2. The main substitute for LCO, which has emerged commercially within the last years, is lithium nickel manganese cobalt oxide. This material belongs to the ternary phase diagram of LiMnO2—LiNiO2—LiCoO2. Additionally this composition can be modified by doping. It is known for example, that elements like Al, Mg, Ti and sometimes Zr can partly replace Co, Ni or Mn. Within the complex ternary phase diagram there is a wide degree of freedom to prepare electrochemically active phases with different composition and quite different performance.
As said above, a high volumetric density is easily obtained with relatively large, dense particles. A high specific capacity can be achieved with a lithium nickel manganese cobalt oxide having a high lithium and manganese content—this material being referred to as HLM, which is an oxide Li-M-O2 with Li:M>1, where M (undoped)=NixMnyCo1-x-y (x≥0, y≥0), and Mn:Ni>>1. The oxide can be conceived as a solid state solution of Li2MnO3 and LiMO2. These compounds are sometimes considered to be nano-composites. A strict distinction between the compounds is not possible because a nano-composite is a solid state solution on atomic scale. Undoped HLM cathode materials have a very high capacity—up to 300 mAh/g. The 300 mAh/g is typically achieved after several activation cycles at a voltage of 4.6-4.8V and discharge to 2.0V. HLM cathode materials generally have a very poor electronic conductivity and slow lithium diffusion, and therefore are prepared as nano-structured or porous powders, making it very difficult to achieve a high tap density.
Generally, for the production of cathode materials with complex compositions, special precursors such as mixed transition metal hydroxides are used. The reason is that high performance Li-M-O2 needs well mixed transition metal cations. To achieve this without “oversintering” (high temperature sintering for a longer period) the cathode precursors need to contain the transition metal in a well-mixed form (at atomic level) as provided in mixed transition metal hydroxides, carbonates etc. Mixed hydroxides or carbonates are typically prepared by precipitation reactions. Precipitation of mixed hydroxides (for example, the precipitation of a flow of NaOH with a flow of M-SO4 under controlled pH) or mixed carbonates (for example, the precipitation of a flow of Na2CO3 with a flow of M-SO4) allows precursors of suitable morphology to be achieved. A problem is the level of impurities; especially, the removal of sulfur is difficult and expensive. The sulphate impurity is suspected to cause (a) poor overcharge stability and (b) contribute to the highly undesired low Open Circuit Voltage (OCV) phenomena, where a certain fraction of batteries show a slow deterioration of OCV after initial charge. Sulphate impurities of up to 5 wt % are measured when using transition metal sulphate solutions in the manufacturing process, both for the precipitation of mixed hydroxides or carbonates.
In this regard, carbonate precursors are known from e.g. U.S. Pat. No. 7,767,189, disclosing a method for preparing lithium transitional metal oxides, comprising the steps of: preparing a carbonate precursor using the following sub steps: forming a first aqueous solution containing a mixture of at least two of the ions of the following metal elements (“Men+”): cobalt (Co), nickel (Ni), and manganese (Mn); forming a second aqueous solution containing ions of CO32−; and mixing and reacting the first solution and the second solution to produce the carbonate precursor, Ni1-x-yCoxMnyCO3; and preparing the lithium transition metals oxide from the carbonate precursors using the following sub steps: evenly mixing Li2CO3 and the carbonate precursor; calcinating the mixed material in high temperature; and cooling and pulverizing the calcinated material to obtain the lithium transition metal oxide, Li Ni1-x-yCoxMnyO2. An aqueous solution containing ions of CO32− is disclosed in U.S. Pat. No. 8,338,037, where a Na containing transition metal composite cathode material is obtained when a (Na—Ni—Co—Mn)CO3 precursor was prepared from a sodium-based carbonate precursor, such as Na2CO3, that undergoes precipitation, followed by simple washing with distilled water, and drying in an air blown oven at about 100° C. The precursors were then mixed with lithium sources and heated at 600° C., followed by re-calcinating at 900° C. in a muffle furnace.
An object of the present invention is to provide carbonate precursors useful as a raw material for producing a lithium nickel manganese cobalt oxide with high lithium and manganese compositions (HLM), where the HLM materials have a relative high tap density and a high capacity upon cycling in a battery.